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Unformatted text preview: previous index next The Doppler Effect Michael Fowler 10/14/09 Introduction ( Flashlet here ) The Doppler effect is the perceived change in frequency of sound emitted by a source moving relative to the observer: as a plane flies overhead, the note of the engine becomes noticeably lower, as does the siren noise from a fast-moving emergency vehicle as it passes. The effect was first noted by Christian Doppler in 1842. The effect is widely used to measure velocities, usually by reflection of a transmitted wave from the moving object, ultrasound for blood in arteries, radar for speeding cars and thunderstorms. The velocities of distant galaxies are measured using the Doppler effect (the red shift). Sound Waves from a Source at Rest To set up notation, a source at rest emitting a steady note generates circular wavecrests: The circles are separated by one wavelength and they travel outwards at the speed of sound v . If the source has frequency f , the time interval between wave crests leaving the source 1 . f = As a fresh wave crest is emitted, the previous crest has traveled a distance , so, since its moving at speed v , , v = The concentric circles represent wave crests generated by the central source at a frequency f waves per second. Their separation is the wavelength , where / f v = , v being the speed of the waves. A stationary observer will (of course) observe them to reach him with frequency f . 2 and therefore . f v = Sound Waves from a Moving Source The Doppler effect arises because once a moving source emits a circular wave (and provided the source is moving at less than the speed of the wave) the circular wave crest emitted continues its outward expansion centered on where the source was when it was emitted...
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